The optimum times for opening and closing the inlet and exhaust valves in an internal combustion engine vary with operating parameters such as engine load and speed. In any engine with fixed angles for opening and closing the valves for all engine operating conditions, the valve timing is a compromise which detracts from the engine efficiency in all but a limited range of operating conditions. For this reason, control systems have been proposed which vary the valve timing during engine operation.
Numerous phase change mechanisms are described in the prior art using a wide variety of techniques and principles. The mechanism which bears the closest resemblance to that of the present invention is that described in the Applicants' own earlier PCT Patent Publication No. WO90/10788.
The latter publication discloses a phase change mechanism for a camshaft of an internal combustion engine, which comprises a drive member to be connected by a toothed belt to the crankshaft, a driven member to be connected to the camshaft and a coupling element for transmitting torque from the drive member to the driven member. The coupling element is connected for rotation with the drive and driven members in such a manner that the phase of the driven member relative to the drive member is dependent upon the position of the coupling element. An inertial member is connected to the coupling element by an eccentric crank which causes the coupling element to move between two end positions when the inertial member rotates relative to the drive and driven members, and means are provided for applying a torque to the inertial member in order to vary the speed of rotation of the inertial member relative to the drive and driven members.
The preferred means for applying a torque to the inertial member in this earlier proposal comprised a one-way clutch acting between the inertial member and one or other of the drive and driven members. Because of torque fluctuations acting on the camshaft, the inertial member will naturally tend to oscillate relative to the camshaft but, when a one-way clutch is interposed between them, relative motion can only occur in one direction and the inertial member will always tend to rotate in one direction relative to the camshaft. The torque will vary with the position of the eccentric crank, being at a minimum in the dead centre positions when the crank offset lies in line with the reaction force and at a maximum when it is at right angles to the reaction force. Though in theory no torque should be acting on the crank in the dead centre positions in practice inertial and frictional effects create enough torque to displace the inertial member from the dead centre positions and the latter will keep on rotating unless it is arrested by latches of some sort. Whenever a phase change is required a latch is released and without any external torque being applied, the inertial member will rotate to its next latched position.